All primates engage in vocal communication. Precise control of the mouth musculature is
very important for the production of species-typical vocalizations. Isolated movements are
known to emerge from a pattern of more general movement. Our study looked at the
differentiation of mouth movements from a more general movement pattern involving combined
mouth and head movements. We studied common marmoset (Callithrix jacchus) monkeys and
performed ultasonography exams to track the degree of overlap between mouth and head
movements through gestation and found that the percentage of overlapping movements
decreased over time. Our findings suggest that mouth movements and head movements are
linked in early gestation but gradually differentiate to become two independent movement
patterns by the end of gestation.